Flip chip mounting of integrated circuit dies on substrates is increasingly used. In one approach, metal bump conductors are formed on the bond pads of an integrated circuit die, the metal bump conductors are arranged for flip chip mounting. Conductive traces are formed on the substrate. Solder is disposed on a portion of the metal bump conductors. The die and metal bump conductors are placed face down (“flipped”) so that the solder contacts the metal traces on the substrate. A thermal reflow process is used that causes the solder to melt and reflow, forming an electrical and physical connection between the metal bump conductors and the substrate. The substrate may be used for a ball grid array package by placing balls on the opposite surface of the substrate.
Because a thermal reflow process is used to reflow the solder, the substrate and the die are subjected to heating. However, as the pitch between the metal traces on the substrate becomes smaller (“fine pitch”) with advances in semiconductor processing, problems can arise when the substrate is heated. Substrate expansion can occur, causing the metal traces to move during the reflow process and solder may form bridging shorts between the traces. The substrate may contract when cooled and cold solder joints or opens may occur. The solder may crack and cause opens.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale.